When burning certain kinds of liquid fuels, especially so called black liquor obtained during the manufacture of cellulose pulp, it is desirable to maintain a constant pressure in the fuel supplied to a spray nozzle in a burner, as this will ensure an atomization of the fluid into droplets of uniform size. A reduction of the fuel supply, for instance during an occasional cut-down of the combustion, is brought about in the prior art by a throttling of the supply conduit. That will, however, result in a reduced pressure, which will alter the spray pattern.